The present invention relates to a process for packaging hermetically semiconductor devices such as diodes and transistors and semiconductor integrated circuits (semiconductor devices and integrated circuits being hereinafter referred to as "semiconductors"), and more particularly to a process for producing thermoplastic packages such as flat packages and dual-in-line packages by putting a lead frame on which semiconductors are mounted between two thermoplastic resin moldings previously molded, at least one of which has a cavity for holding the semiconductors, and the sealing the molding into a body.
Hitherto, as a process for packaging semiconductors there are known the packaging by hermetically sealing with a metal, ceramics or glass and the packaging with plastics, and in recent years inexpensive plastic packages are the main current because of the increase of the reliability by the progress in technique for protecting chips.
The plastic packages put into practical use at present are classified into two kinds according to processes of the production, i.e. the so-called transfer molding process and casting process. In particular, there have been widely employed the plastic packages produced by a transfer molding process using an epoxy resin which has excellent characteristics such as close adhesion to metals, moisture resistance and electric and mechanical properties.
As shown in FIG. 1 which is sectional view of a package according to a conventional molding process for packaging semiconductors, in such a molding process a semiconductor chip 2 mounted on a tab 1 is interconnected to leads 4 of a lead frame 5 by metal wires 3 such as gold and aluminum, the lead frame 5 is put between an upper mold 6 and a lower mold 7 and a molding resin material is filled in a cavity 8 through a sprue, a runner and a gate of a transfer molding machine (not shown) and is molded.
This molding process has the disadvantage that a gap is formed between the molds and the lead frame due to ununiformity in thickness of the lead frame, accuracy of mold processing, and abrasion and deviation in dimensions of the molds accompanied by the use, and in and around this portion a flash is often formed. Since such a flash on the leads of the lead frame results in poor contact between the leads and a socket and prevents a soldering operation, a deflashing is required. The molding process has a further disadvantage of producing an effect on the reliability of semiconductor functions in some cases, since the chip is in the state of high temperature and high pressure during molding due to direct contact with the resin material.
In order to improve such disadvantages of the molding process, there has been proposed a process as disclosed in Japanese Patent Unexamined Publication No. 77669/1977. As shown in FIG. 2 which shows a sectional view of a package, in this process a lead frame 5 with no semiconductors thereon is put between a plastic case 11 having an opening portion 9 and a rib 10 to be welded which has been previously formed from a thermoplastic resin, and a plastic case 12 having a bottom portion. The cases are joined in a body by means of ultrasonic or high-frequency welding and a semiconductor chip (not shown) is then placed therein through the opening portion 9 of the case 11.
However, this process has the following disadvantages. It is necessary to attach the chip within the package through the opening portion 9 of the case 11 and then to interconnect the circuit of the semiconductors to the leads with metal wires and, therefore, the wiring operation is often conducted with difficulty, since the plastic case hinders the operation. Even if the operation is smoothly conducted, there is further required a step of hermetically sealing the opening portion 9 of the case 11 with another cover or a resinous material. Moreover, there is a problem in the reliability of hermetical sealing, since there are two portions to be joined, i.e. the portion between the cases and the leads and the portion between the case and the cover or between the chip and the resinous material.
In order to eliminate such disadvantages, the following process for packaging semiconductors has also been proposed. FIG. 3 shows a sectional view of a package obtained according to such a conventional process. Two thermoplastic moldings 13 and 13' previously molded, at least one of which has a cavity for holding semiconductors and between which a lead frame 5 with semiconductors (not shown) is interposed, are placed between a fixture 14 and an ultrasonic vibration horn 15. Ultrasonic vibration is imparted to the thermoplastic moldings 13 and 13' so as to cause the resin made molten by the vibrational friction to flow between the thermoplastic moldings 13 and 13' and into lead gaps 16 of the lead frame 5.
This process also has the following disadvantages. In the contact friction between a flat joining surface of the molding and the lead frame, the contact area is relatively large and a large energy is required to melt the resin. Accordingly, when the ultrasonic vibration time is made constant, the amplitude of the horn must be increased, and on the other hand, when the amplitude is made constant, the vibration for a long period of time is required. These may cause fatal problems in hermetical sealing due to the incorporation of air into the molten resin during vibration, which makes the joint porous, and which leads to deformation and breaking of the delicate leads and bonding wires due to the increase of the amplitude of ultrasonic vibration or the increase of the vibration time.